If alcohol is out of your system after 48 hours. Can they run a test to see if you have drank in the last five days? How can they test for something that's out of your system already?

Ethanol is metabolized in the liver (primarily) and during this process specific by-products are created. These products are created only from metabolizing ethanol and remain in your system long after the ethanol is gone.

The presence of these metabolites, mostly Ethyl Glucuronide (EtG) and/or Ethyl Sulfate (EtS) is accepted as evidence that alcohol was present within a short time previous.

It would be very unusual for EtG/EtS to be present at 4 days post-consumption. However EtG has been detected for as long as 133 hours in chronic alcoholic with liver damage. Generally, even a heavy drinking episode will clear between 60-70 hours. As further example after one standard drink the resultant EtG will be eliminated in 20.6 hr (ave-range 3-30 hr). As a comparison ethanol in your system would be detectable for only 3 hrs max)

Your first statement is not really accurate " after 48 hours". You fully metabolize alcohol after 6 hour after you stop drinking. It really depends on the type of test given. Blood alcohol tests may only be good for about 6 hours after the last drink but urine tests can be valid after 80 hours, that's less than 4 days. Why do you ask?